8 × 10 ft bathroom: 80 CFM
The floor area is 80 sq ft. At 1 CFM per square foot, the target is 80 CFM. It exceeds the 50 CFM floor used here.
Bathroom fan size calculator
Enter bathroom dimensions to find an HVI exhaust-airflow target in CFM. For rooms over 100 square feet, add the fixtures.
Area for small rooms, fixtures for large rooms
Use the area rule through 100 square feet and the fixture rule above that size. The result shows the applied rule for a standard-ceiling bathroom with a fan used during and after bathing.
| Bathroom | Airflow guide | What to check |
|---|---|---|
| Up to 50 sq ft | 50 CFM minimum | Choose a certified rating and verify installed airflow. |
| Over 50 through 100 sq ft | 1 CFM per square foot, rounded up to a whole CFM | Room area includes the shower and tub. |
| Over 100 sq ft | 50 CFM per toilet, shower or regular tub; 100 per jetted tub | Do not count one jetted tub twice. |
| Higher ceilings or special ventilation | Standard-ceiling reference only | Have the target and ventilation arrangement checked. |
Two examples you can reproduce
12 × 12 ft bathroom: 200 CFM
The area is over 100 sq ft. One toilet, one shower and one jetted tub give 50 + 50 + 100 = 200 CFM. Do not add another bathtub count for the same jetted tub.
Why the rated CFM is not the installed airflow
A long, restrictive or poorly installed duct can reduce exhaust airflow. Use the fan manufacturer's pressure and duct data to check the installation. This calculator does not predict a duct diameter, compensate for elbows or certify a local-code requirement.
Exhaust the air outdoors, provide replacement-air access, and plan ventilation separately for an enclosed toilet room. HVI recommends running the fan for 20 minutes after bathroom use. Steam showers need their own ventilation arrangement.
Bathroom fan sizing questions
Why not just use eight air changes per hour?
HVI describes roughly eight air changes per hour as a useful guide, but its consumer table specifies area and fixture rules. This calculator uses those table rules and shows which applies instead of silently switching to a volume formula.
What happens at exactly 100 square feet?
The small-room rule still applies: 100 CFM for a standard ceiling. A room larger than 100 sq ft uses the fixture-count rule.
Can I choose a continuous fan from this result?
No. Continuous exhaust uses a different ventilation plan. This tool's stated scope is intermittent bathroom exhaust.
Sources and scope
Primary guidance checked October 12, 2026.
- HVI Bathroom Exhaust Fans: consumer guide
Source for this calculator's 50 CFM minimum, area rule, fixture allowances and higher-ceiling caveat.
- HVI Bathroom Ventilation
Fixture-method worked example, fan placement and post-use operation. Its 40 sq ft example gives 40 CFM; this calculator deliberately uses the consumer guide's stated 50 CFM minimum instead.